Randy: Can we, for the purposes of discussion, define varroa virulence as its ability to invade and reproduce in colonies of honey bees?



Me: I don’t know how deeply folks want to go into this. The term “virulence” itself may be the cause of more confusion than enlightenment



> Virulence: the most commonly used definition of virulence in evolutionary biology is the cost to host fitness inflicted on a host by a parasite. It is important to note that other mortality-based definitions exist that differ from absolute parasite-induced host mortality rate, including case mortality, expected time until death from infection, and lethal dose.



> Inconsistencies in the definition of virulence may also play a role in the observed discrepancies. In evolution experiments the range of measures of virulence reflects the variety of definitions, from differing measures of mortality, fitting more closely with the definitions used by modellers, to more comprehensive fitness-based methods better aligned with definitions often used by evolutionary biologists. 



> Information on virulence evolution is critical for understanding disease dynamics. Theory predicts that under certain evolutionary conditions virulence should increase; for example, during host–parasite coevolution. Although these theoretical predictions are supported by natural observations, tests of these hypotheses using experimental evolution have yielded confounding and contradictory results, with discrepancies often being seen among experiments. 

> 

> Thus far, however, although experiments using bacterial hosts have generally found results to be consistent with theory, in experiments using eukaryotic pathogens predicted effects have not been seen or have differed from observations in nature.  We emphasise the key role of parasite transmission mechanisms that can explain many of the observed discrepancies among evolution experiments.



Rafaluk, Charlotte, et al. "When experimental selection for virulence leads to loss of virulence." Trends in parasitology (2015).



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